The present invention relates to the manufacture of generally rectangular concrete tiles having a top side (in the manufacturing process proper) the configuration of which is defined by rectilinear motion of a selected surface generatrix between two opposite edges of the tile, and a bottom side (likewise in the manufacturing process proper) the configuration of which differs from that of the top side at least as a result of a thickness variation of the tile in the direction of motion of the generatrix of the top side thereof.
A typical example of a concrete product of this kind is a roofing tile, the top side of which is most frequently either single- or double-cupped, and the bottom side of which follows the cross sectional contour of the top side in part but at certain places along its length is formed with transversally extending ridges and frequently also with local knobs. However, there are also many other types of concrete products presenting fairly similar manufacturing problems and among them certain types of wall facing tiles, covering slabs, etcetera may be mentioned.
More specifically the invention is concerned with a process and a machine for manufacturing in large series concrete tiles of the kind defined hereinbefore, in which portions of a still but yet plastic concrete mass are deposited under heavy compaction on separate molding trays each having a bearing surface which is complementary to the bottom side of the tile to be formed, the compaction being carried out by placing each tray in a predetermined position on a movable tray carrier and passing the latter with the tray thereon under a rotating pressing roller, the circumferential surface of which has a generatrix which is complementary to the suface generatrix of the top sides of the tiles, and in which each concrete mass portion thus molded and compacted is retained on its related tray until having reached a self-supporting state in which it may be readily handled without the support of the molding tray.
It is well known that by using such a process a considerable number of identical concrete tiles may be molded per hour, but also that, so far, no appreciable reduction of the manufacturing cost per product unit has been reached in comparison with the cost for manufacturing the tiles much more slowly by using more primitive mechanical equipments and a considerably higher amount of manual work. The reason for this is the very great investment which in the case of mass production has to be made in the molding trays which have to follow the tiles from the time, when they are molded, and all up to the time, when the concrete mass has hardened sufficiently to make the products capable of resisting all such strains to which they will be subjected during their continued handling in the factory as well as during their transportation to possible retailers or directly to consumers.
During decades various attempt have been made to shorten the time in which the molded concrete mass needs to remain on the molding trays, but with few exceptions the success of such attempts has been doubtful. Nowadays it is common practice, however, to accelerate the hardening of the molded concrete products in special chambers, in which favorable temperature and moisture conditions are maintained artificially, and such a measure is preferably used also in combination with the present invention, but neither in this way any substantial reduction of the manufacturing costs has been achieved, when the production capacity of the plant is, for example, several thousands of units during the period of time corresponding to the circulation time of the individual molding trays.
On the other hand, the tiles manufactured according to the process defined hereinbefore are, no doubt, of a much superior quality as compared with those produce in a more primitive way. In fact, their mechanical strength is considerably improved and, above all, their tendency to absorb moisture and to crack or burst in cold weather is appreciably reduced. This is a result of the heavy compaction of the concrete mass ensured by the rapidly rotating pressing roller which, in addition, by a kind of rubbing action condenses and seals the surface of the product engaged by the roller itself. It is to be noted, however, that the compacting forces exerted by the pressing roller also subject the molding trays and their carriers to severe strains, a pressure on the concrete mass, and hence, on the trays, of more than 100 MPa not being unusal.
The molding trays which have been used so far in the process defined hereinbefore were made of metal castings and, depending on the accuracy of the casting technique used, needed more or less extensive finishing work before they could be used. As a result, such molding trays were very expensive, but nevertheless trays of this kind were considered indispensable, because by the provision of stiffening fins or ridges on their lower sides they could easily be given the required strength to resist deformation under the considerable loads referred to above. However, the high unit price of these cast trays evidently results in heavily increased investment costs in a plant for the mass-production of the concrete tiles and thus counteracts the reduction of manufacturing cost gained by the high production rate. In addition, the necessary frequent replacement of the molding trays, which are subjected to a significant wear in use, causes considerable extra costs.
Now, a general object of this invention is to improve the economy in the mass-production of concrete tiles and similar concrete products according to the process defined hereinbefore.
With this object in view the invention is based on the understanding that the investment cost in a plant for mass-production of concrete tiles, and hence the cost per produced unit, could be drastically reduced by using a simplier and thus considerably cheaper kind of molding trays then the one described above, even if this would result in a certain but moderate cost increase as far as the machine equipment proper is concerned. Molding trays which would be excellent for such purpose are those made of sheet metal of only a moderate thickness not exceeding about 3 millimeters, because such sheet metal trays, when made in reasonably large series, may easily be given even farily complex surface configurations by deep drawing and/or local embossing and will be much less expensive than the cast trays referred to hereinbefore. Besides, such sheet metal trays have already been successfully used in certain older tile making processes and machines, in which only a moderate compacting of the concrete mass, such as by vibrating, was resorted to. A further advantage of sheet metal trays is that they may be made of stainless sheet steel, which will make them fairly resistant to both wear and corrosion.
However, the problem with such sheet metal molding trays is that, unless special measures are taken, they will not be able to take up the heavy deforming loads deriving from the pressing roller used for compacting the stiff concrete mass on the trays in modern tile molding processes and machines of the kind referred to hereinbefore. In fact, the sheet metal trays, when used in a conventional manner, will at least resiliently yield and frequently give away and become permanently deformed under such loads, which, of course, is entirely unacceptable.